This invention relates to a method of and an apparatus for lithographic projection of a mask pattern on a plurality of fields of a wafer by measuring the height of the wafer surface prior to exposure.
U.S. Pat. No. 6,208,407 has described such a system having a projection station and a measuring station. While a first wafer on a first stage is undergoing an exposure process in the projection station, a first height sensor maps the surface of a second wafer on a second stage in the measuring station, and a second height sensor measures the height of the second stage. After the exposure process is completed, the first stage is removed from the projection station, the exposed first wafer is removed from the first stage and the first stage is provided with a new wafer and then moved to the measuring station. In the meantime, the second stage and the second wafer are removed from the measuring station and placed in the projection station. The projection station is provided with a third height sensor. The second stage, after brought into the projection station, has its height measured by the third height sensor to ascertain whether it is at the ideal height for the exposure. In other words, three height sensors are involved per exposure of a wafer, and this will compound the errors leading to a loss of precision. In order to properly initialize this sensor, still another sensor may be required. Any error in such an additional sensor will add to the initialization error of the third sensor, causing further degradation of precision.